The background of the invention will be set forth in two parts.
1. Field of the Invention
This invention relates to surface acoustic wave devices and more particularly to delay lines.
2. Description of the Prior Art
A surface acoustic wave delay line generally comprises a pair of transducers, one input and one output, disposed on a piezoelectric substrate surface capable of supporting surface acoustic wave energy propagating between the transducers. Basically, the time delay or insertion phase provided by the device is governed by the distance between the transducers and the average velocity of the propagating surface wave energy. That is, the longer the distance and/or the lower the velocity, the longer the time delay and the larger the insertion phase.
In the past, it has been difficult, time consuming, and expensive to precisely provide a desired time delay in such devices. For example, in accordance with one prior art technique, the spacing between transducers is modified by either redesigning the photomask and fabricating a new device, or one of the transducers is eradicated and redeposited with a new, shifted position. Another technique is to deposit a continuous metal film of appropriate length to increase (only) the delay. Both techniques require complete disassembly of the delay line and photolithographic processing. These are generally considered to be "last resort" measures because of the high cost of fabrication.
An area in which a precise time delay is highly desirable is that of surface acoustic wave oscillators. Such oscillators consist of an amplifier with a delay line in its feedback path. The frequency of oscillation is a critical function of the time delay which must be typically controlled to within two parts in 10.sup.6. Currently, frequency adjustments are made by introducing appropriate phase shifts in an external network because the required control cannot be achieved in a conventional delay line.
In fact, there are an ever increasing number of applications of surface acoustic wave delay lines which require precise control of time delay or insertion phase. A few examples of these applications are: time-multiplexing delay lines and multiple correlator filters where extreme time delay accuracy is required. Here, fundamental limitations imposed by such factors as crystal orientation tolerances make it difficult and expensive to achieve the required accuracy, especially when a batch of identical lines are required. It should therefore be evident that a simple, accurate and inexpensive technique that would provide the desired time delay or insertion phase, and that would make it possible to easily compensate for errors in a plurality of identical lines, for example, would constitute a significant advancement in the art.